Set inside a clear casing, the same shape as the diamond, the
basketball-sized, man-made jewel (it took six months to process)
rotated in circles providing energy and processing power to the many
circuits in the Bat Computer.

On episode No. 57/58 "Marsha, Queen of Diamonds"/"Marsha's Scheme
With Diamonds" (aired 11/23&24/1966) Marsha, Queen of Diamonds
(Carolyn Jones) attempted to locate the Bat Cave and steal the Bat
Diamond for her own collection.